Aluminum plating employing amine complex of aluminum hydride



United States Patent ALUMINUM PLATING 'EMPLOYING AMINE COMPLEX OF ALUMINUM HYDRIDE David R. Carley and James H. Dunn, Baton Rouge, La.,

assignors to Ethyl Corporation, New York, N.Y., a corporation of Virginia d V v I I N Drawing. Continuation-impart of appliafionsSer. No. 255,662, Feb. 1, 1963, and Ser. No. 243,693, Nov. 16, 1962 which are continuationIs-in-part of application Ser. No. 25,838, May 2 1960. This application Sept. 22,

" 1966, Ser. No. 581,187

17 Claims. (CL 117-1072) ABSTRACT OF THE DISCLOSURE A process for depositing an aluminum coating on a substrate by contacting the substrate at an elevated temperature with 'thermally decomposable vapors of an amine complex of aluminum hydride. Diffusion of the coating may be efiected into the substrate without intervening treatment.

This application is a continuation-in-part of applications Ser. No. 255,662, now abandoned, filed Feb. 1, 1963, and Ser. No. 243,693, now abandoned, filed Nov. 16, 1962; both of which are continuations-in-part of application Ser. No. 25,838, filed May 2, 1960, and now abandoned.

The present invention relates to plating of aluminum on substrates by the decomposition of a complex aluminum compound, and to preparing improved high temperature aluminum coated alloys.

Aluminum is a very desirable coating material, particularly for oxidizable metals. The surface of aluminum exposed to air rapidly develops an oxide film that protects the remainder of the aluminum from oxidation. This is considered to be attributable to the relationship between the crystal structures of aluminum and its oxide.

The oxide crystals fit in place so compactly over the aluminum crystals that, notwithstanding the highly reactive nature of aluminum, the oxide film very effectively seals the bare metal against exposure.

Unfortunately, aluminum is a difiicult metal to apply in the form of a coating, and at elevated temperatures it will diffuse into the metal substrates, tending to Weaken them unless the amount of diffusion is extremely limited. Heavy coatings of aluminum are therefore not desired. Heavy coatings also produce dimensional changes that complicate production.

Among the objects of the present invention is the provision of novel coating techniques and coated products which are relatively simple to use'and make, and by which very effective and highly uniform aluminum coatings are produced.

The foregoing as well as additional objects of the present invention will be more fully understood from the following description of several of its exemplifications.

It has been discovered that excellent deposits of aluminum are very simply formed by vapor plating from an amine complex of aluminum hydride. Such deposits are strongly adherent, can be made quite thin, and are highly uniform and pure, so that they can be subjected to a diffusion operation without intervening treatment.

The vapor plated aluminum produced in the foregoing 3,3 75 Patented Mar. 26, 1 968 manner has no detectable carbon content and is, more over, exceptionally pure. Carbon contamination is a chief source of concern in many vapor 'plated coatings inasmuch as the compounds used to provide the coating vapor generally. contain carbon, and the deposition that forms the desired coating metal also forms carbon or canbides that tend to deposit with the metal. The coatings of the present invention are over 99.9 percent pure although they are deposited from compounds that contain carbon. The corrosion resistance of an aluminum surface is well known to increase markedly as the aluminum increases in purity. V

x The amine complexes of the present invention also deposit the desired coatings very smoothly and uniformly. This is somewhat surprising inasmuch as uncomplexed aluminum hydride is known to decompose explosively in 9 vapor-plating processes, and it might normally be expected that the complexed hydride of the present invention would at least partially decompose to form the explosive uncomplexed hydride.

It is another feature of the present invention that the aluminum coatings it provides are of superior quality, judging from comparisons made by the industry with other available coatings. Thus, the consesus of all evaluations made by the industry is that coatings applied by the process of the present invention exhibit overall superiority over other established aluminum coatings known at the time of the evaluations. Specifically, the evaluations have shown such coatings to be very pure, adherent to the substrate, ductile, amenable toanodizing, uniform in temperatures lower than heretofore found desirable in the aluminum vapor plating art.

For example the 1962 Government report Vapor Plating 'of Aluminum on Steel by Mr. J. J. Crosbyof the Aeronautical Systems Division, Wright-Patterson Air Force Base (Government Report ASD-TDR-62-907),

indicates that fairly pure aluminum of very low porosity can be deposited on mild steel and other materials by thermally decomposing vapors of triisobut'ylaluminum using a mixture of argon and isobutylene as a carrier gas to suppress undesirable side reactions. To achieve such results, the author discloses that the isobutylene content of the carrier gas must be carefully controlled and also that it is necessary to employ a substrate temperature of at least 220 C. On the other hand, the instant process is preferably conducted at temperatures below 200 C., in

fact as low as about C. Thus, the present invention makes it possible to produce very uniform, adherent, non-porous, and highly pure aluminum coatings by vapor deposition on many substrates heretofore not amenable to other aluminum vapor deposition techniques. Employing the novel process of the instant invention a'sdiscussed supra and as illustrated in the examples set forth hereinafter, substrates such as cotton, khaki, rayon, plastics, and the like have been aluminum coated. The coatings on these materials have been found to be' extremely pure and very adherent, and it withstands repeated flexing.

Another feature and significant advantage of the present invention, unlike the most preferred vapor deposition techniques today employing alkylaluminum compounds, is that it is not necessary to employ a carrier gas or any other special gas to suppress undesirable side reactions that occur in the pyrolysis of alkylaluminum compounds. For example, in US. Patent 2,990,295, col. 2 beginning at line 1, the necessity of employing a carrier gas, such as argon, is disclosed. According to the patentees, it was necessary to have an oxidizing agent present at the plating site to keep the carbon content of the deposit below 0.5 percent. On the other hand, pursuant to the practice of this invention, reproducible coatings having a purity of 99.99 percent have been realized without employing either a carrier gas or any other special provision.

To illustrate these important results, the following tests were made. A number of copper samples were aluminum coated by the present process and, along with identical uncoated samples, were analyzed for total carbon content by the highly sensitive leco carbon method. The results within the range of experimental error were about 0.2 milligram carbon per sample, both for the aluminum coated and the uncoated samples. These results indicate that no detectable amount of carbon was added with the coating.

Another feature of the instant invention is its commercial attractiveness. By this is meant the present process greatly minimizes the cost of plating equipment as well as its operation. For example, by not having to employ critical proportion of special process gases, the awkwardness and cost of the extra complications (storage vessels, flow proportioning equipment, etc.) is avoided. By operating at low temperatures, it is also possible to effect substantial savings in the cost of substrate heating apparatus. For example, it is a simple matter to heat a substrate to preferred coating temperatures of the present invention with a few heating lamps. In short, the present process is much more expeditious than priorpractical aluminum vapor plating processes.

The special vapor plating compounds employed herein have been known in the prior art (Wiberg et al. publication entitled About Monomeric Aluminum Hydride All-I Z. Anorg. Allgem. Chem. 272, 221-32 [1953], but have been overlooked by the vapor plating industry. Moreover, the art seems to have cautioned against usingsuch compounds. For example, in US. Patent 3,041,197, col. 1,

line 26, it is disclosed that the use of aluminum hydride as a plating agent is highly unlikely because of the explosive and sensitive nature of that compound. Additionally, in the article entitled Gas Plating Solves High Temperature Problems by R. H. Chandler, Industrial Finishing (London, vol. 14, pages 77-78 (1962) it is disclosed that plating with aluminum compounds containing hydrides or tending to form hydrides thereof produces deposits of poor quality unless a reactive olefin is introduced into the system. In short, the efforts in the art have been to avoid use of aluminum hydride type compounds. Contrary to these teachings, the present process employs an aluminum hydride to produce outstanding coatings.

In the above cited Wiberg et al. paper, the authors state that (page 226) upon rapid heating to about 100 C. a coating compound of the present invention becomes cloudy, indicating that polymeric aluminum hydride was being formed. Upon heating the starting compound with a free flame, the authors observed the formation of an aluminum mirror at the hot spots. These reports tend to indicate that the high temperature of a free flame is needed to decompose the amine complexes to aluminum, and that at lower temperatures they decompose to the uncomplexed hydrides which is elsewhere said to be unsuitable because of explosive tendencies. These could be the reasons why the art has heretofore not attempted to use these Wiberg et al. compounds for vapor plating.

Many other organometallic compounds are known to decompose and deposit their metals. This feature alone is not sufficient to qualify the compound for practical vapor plating operations. Suitable compounds must possess certain critical properties; namely, they must be of moderate volatility at temperatures below decomposition temperature (i.e., be capable of volatilization without decomposition), and they must also decompose at a moderate temperature since excessively high decomposition temperatures cause interaction of the deposited metal with the substrate, in the case of metal substrates, to produce an alloy instead of a simple coating. It is evident that the Wiberg et al. teaching of spotty free-flame deposits of undefined nature suggests none of these critical properties.

Certain conditions, some of which have already been mentioned, favor the practice of the present invention. The objects of the present invention can be attained by heating a substrate to a temperature above 120 C., generating vapors of an amine complex of aluminum hydride in a non-oxidizing atmosphere, contacting said heated substrate with said vapors in a non-oxidizing atmosphere, maintaining the temperature of said substrate above about 120 C. while continually contacting it with said vapors until the desired coating thickness is achieved, and thereafter discontinuing contact between said coated substrate and said vapors.

When it is desired to utilize the above process to provide an improved high temperature metal having an alloy surface comprising aluminum and a base material having as its principal constituent a metal selected from the group consisting of cobalt, nickel, iron, niobium, tantalum, and titanium, the above process is practiced with the substrate beinga high temperature alloy base material, and in addition to the above process there is practiced the step of heating the aluminum coated base to a temperature above 1000 F. to diffuse the coating into the base material.

It is preferred that the amine complex of the plating agent contain as the amine constituent thereof a tertiary alkyl amine, especially wherein each alkyl group contains 1 to 6 carbon atoms. These are preferred since they pyrolyze very smoothly and cleanly to leave a deposit having a high degree of purity. The trimethylamine and bis-trimethylamine complexes of aluminum hydride are especially preferred because of their excellent volatility and stability.

Whenutilizing the trialkylamine complexes of aluminum hydride, best results are obtained within the temperature range of from about 120 C. to about 250 C., especially between about 160 C. to about 200 C. Coatings efi'ected at this temperature range are characterized by their exceptionally high adhesion and cohesion.

To effect vaporization of the plating compound, it is preferable to do such in a non-oxidizing environment at a temperature within the range of from about 20 C. to C. It is also preferred to conduct the instant process under a reduced pressure within the range of from about 0.3 to 30 millimeters of mercury. Under these conditions it is much easier to insure an inert plating atmosphere as well as control the vapor flow rate which in turn largely dictates the plating rate for a given operation. Hence, a preferred embodiment of the instant invention is a vapor deposition process for plating a substrate with aluminum by heatingthe'substrate in a non-oxidizing environment to a temperature within the range of from about C. to about 250 C., generating vapors of a trialkylamine complex of aluminum hydride in a non-oxidizing environment at a temperature of about 20 C. to about 80 C. which is insufiicient to effect significant thermal decomposition thereof, contacting said heated substrate with said vapors ina non-oxidizing environment, maintaining the temperature of said substrate within said range while continually contacting it with said vapors until the desired thickness of coating is achieved and thereafter discontinuing contact between said coated substrate and said vapors and allowing said coated substrate,

to cool in a non-oxidizing environment. It is understood that the first two steps, above, may be conducted in any order or concurrently.

When producing a high temperature alloy it is preferred to coat the base materials at aratewithin the range of from about 0.5 to about 20 mils per hour since within this range it is easier to correlate the operating parameters. Notwithstanding the specific physical properties possessed by a base material, it is preferred to diffuse a coating prepared as above at a temperature above 1000 F. and preferably within-a'range of from about 1200 F. to about 2250 F. Diffusion within. this temperature range, at times varying from /2 to 16 hours,

produces a coating having the'requisite degree of oxidation resistance and uniformity for 2100 F. temperature applications.

In a highly preferred embodiment of this invention an improved high. temperature alloy having an alloyed surface com-prising. aluminum and a base. material having as its principal constituent a metal selected from the group consisting of cobalt and nickel, is prepared by a process including heating said base to a temperature within the range of from about 160 C. to about 200 C., contacting said base at a reduced pressure of less than 10 millimeters of mercury with a plating. gas prepared by vaporizing a solution of a trialkylamine complex of aluminum hydride dissolved in a hydrocarbon solvent, continually contacting said heated alloy base material with said plating gas at said reduced pressure at a plating rate of from about 0.5 to about mils per hour to produce a coating of from about '1 to about 5 mils thickness of essentially pure aluminum, and thereafter heating said aluminum coated alloy base materialto a temperature within the range of from about 1200 F. to about 2250 F. for a period of time within the range of from about /2 to about 16 hours to diffuse said essentially pure aluminum coating into said alloy base material.

Having thus set forth the invention, the following examples are presented to illustrate the practice and beneficial results of the instant invention. All parts are by weight unless otherwise indicated.

EXAMPLE I Plating apparatus The plating chamber was a cylindrical vessel having a, removable cover or door at one end, its other end be.- ing enclosed by a dished head constructed integralwith the walls of' the vessel. Extending through the dished head of the vessel along its center line Was a tubuluar member having apertures in open communication with the interior of the vessel whereby a source 'of vapors could be fed therein. The tubular member was piped to vapor generation means. The vapor generation means was a cylinder wherein the plating compound was placed, and a temperature-controlledoil bath in which the cylinder was partially immersed. A flow meter and flow control valve were positioned in the piping connecting the tubular member in the plating, chamber with the vapor generation means.

Substrate retaining means in the form of-a wire screen tumbling basket was positioned in the plating chamber around the openings of the tubular member. The tumbling basket was positioned upon rotating-means which extended through the dish head to driving means. For heating the substrates contained within the tumbling basket, electric resistance radiant heaters were positioned inside the plating chamber in close proximity to the tum: bling basket. A vaporoutlet means on the plating chamber was connected to vacuum inducing means.

Thermocouples were positioned in close proximity to the substrates being heated whereby their temperature could be observed and'controlled'throughout the plating operation.

Plating procedure After cleaning, and deburring. where necessary, the

parts to be plated were loaded into the plating basket as rapidly as possible to minimize oxidation of their surface by air. The plating chamber was then sealed and;

, into the system.

Rotation of the basket wase started and heat was applied until the parts reached a platingtemperature of about C.'The heat load was; then adjusted to main-.

tain a constant temperature.

While the system was being heated, the plating compound was charged to the vapor generation means under an atmosphere of nitrogen. In the following runs the plating compound was a solution of the trimethylamine complex of aluminum'hydride in toluene.

The solution was then heated to generate the requisite plating gas which comprised vaporous trimethylamine complex of aluminum hydride and vaporous toluene.

To start the plating operation, the chamber was opened to the vacuum pump and condenser and the plating compound control valve was opened. During plating the last-mentioned valve was adjusted according to the plating chamber pressure and volumetric change in the feed vessel which was transparent and calibrated so that consumption of the plating compound could be readily determined.

A knockout pot wasprovided in the conduit between the plating chamber and the vaporizer to remove entrained materials from the plating gas. The piping system between the plating chamber and the vaporizer was also heated during the operation to prevent condensation of the plating gas.

During plating, the temperature inside the basket was maintained at l70i-10 C. by adjusting the heat load applied to the heaters mounted around the tumbling basket. Additional heat was required upon startup of vapor flow because of the heat absorbed by the vapors.

The plating period ranged from 30 to 60 minutes depending upon the basket rotational speed selected and the rate of compound feed. After the flow of plating compound was completed, the feed system was flushed for about 5 minutes with dry toluene to remove residual plating compound. Simultaneously, the heat was cut off to permit cooling of the parts. Basket rotation was continued during the initial cooling period to prevent overheating of parts adjacent to the radiant heaters.

Evaluation of coatings Samples of the coated parts were selected at random for evaluation according to the following tests:

(1) Appearance.-Coatings were examined visually and under a miscroscope for color, roughness and galled of nicked areas.

(2) Ductill ty and adherence.Several samples were picked and scraped with a scalpel under a microscope to obtain a qualitative indication of ductility and adherence.

Tensile tests, using adhesives, were. applied to flat head The following runs were made:

Run 1 Run 2 Parts Plated:

Description M x 1 national fine x 2% corrugated and M x 3 coarse steel. Huck bolts. threaded steel cap screws.

Surface Preparation pickling, grit Liquid honing.

s Operating Data:

Basket, r.p.m 0.5 0.5. Plating time, rnin 60 60. Plating agent feed rate, cc.lrnin 4. 6 2. 25. Temperature C.:

Inside Basket 162/172 170/173. Pressure, mm. Hg 1.7/2.2 1. 0.

Coating Evaluation:

Appearance Smooth Smooth. Color White, some gray Gray, smooth bronze.

blue areas. Adherence Good Good. Ductility Soit Soft. Corrosion Resistance,

ays: In distilled H 18 21.

avg. In percent salt 9 4.

spray, avg.

Run 3 Run 4 Parts Description M x 1% steel TQ, x 15 steel TQ Bolt Bolt Surface Preparation.-. Degrea se, vapor blast, Degrease, vapor blast.

H10 rinse acetone Hi0 rinse, acetone rinse, N1 dry. rinse, N1 dry. Operating Data:

Basket, r.p.m 0.5 0. 5. Plating time, min..- 60 60. Plating agent feed 8.0 7.8.

rate, cc./min. Temperature 0.: 170-180 165-183.

Inside basket. Pressure, mm. Hg 1-3 1-4. Coating Evaluation:

Adherence Good Good. Appearance Smooth, white-gray.-- Smooth, white-gray.

Corrosion Resistance ests: In 20 Percent Salt pray:

Avg. survival,

a Days/mil 40.. In Distilled Water:

in. Surface Preparation Degrease, vapor blast, Dcgrease, vapor blast,

H2O rinse, acetone H O rinse, acetone rinse, N; dry. rinse, N1 dry. Operating Data:

Basket, r.p.m 0.75-... 0.8. I Total time, min 40.. 40.

Plating agent ieed 9. 5.- 7. 5.

rate, cc./min. Temperature 0.: 170/180. 180/200.

Inside basket. Pressure, mm. Hg..- 2/8 2/3. Coating Evaluatro erence Good Good. Appearance Smooth, white Smooth, white. Corrosion Resistance ests: In 20 Percent Salt pray:

Avg. survival, 7. 4 7. 4.

days. Days/mil 39 32. In Distilled Water:

Avg. survival, 11 20.

days. Days/mil 58 87.

Run 7 Parts Description M x 1% steel HL-Lok Pin.

Surface Preparation Degrease, vapor blast, H O rinse, acetone rinse,

Run 7Continued Corrosion Resistance Tests:

InS20 Percent Salt In Distilled Water:

Avg. survival, 75.

days. Days/mil 200.

The purity of the above coatings was exceptionally high, above 99.99%. In addition to their exceptional corrosion resistance, these high purity coatings have better workability and when diffused in high temperature sub- 15 strates produce very desirable products.

Diffusion is preferably but not necessarily eifected in an inert atmosphere such as hydrogen, nitrogen, and the like to prevent oxidation and contamination of the coating during diffusion.

To demonstrate this benefit of the present invention, the following examples are presented. High temperature alloys were utilized as the substrates on which the unique coating of this invention was applied. The coating was then diffused and oxidized.

EXAMPLE II Plating apparatus radiant heaters, high frequency induction heating means were mounted around the plating vessel in the zone Wherein the substrates were positioned when mounted. The top of the vessel was provided with a vapor inlet comprising means extending into the vessel in close proximity to the mounted substrates. An outlet was provided at the top of the plating chamber whereby the by-products of decomposition were removed during the plating operation. Another inlet means was provided at the bottom of the vessel to facilitate purging.

Plating procedure After cleaning the high temperature alloy substrates, they were mounted on the rotating substrate means within the plating chamber which was thereafter sealed. These steps were conducted as rapidly as possible to minimize contamination and oxidation of the substrate surfaces. The plating chamber was then evacuated to less than 10 millimeters Hg. At this point nitrogen was injected into the plating vessel to purge it of any residual air retained therein. The plating chamber was then re-evacuated to the respective pressure shown below in the various runs. During this step and throughout the plating operation, constant vigilance was maintained to make certain that air was not leaking into the system.

Rotation of the substrates was started and heat was applied until a plating temperature of about 180 C. was attained. The heat load was then adjusted to maintain the constant temperature shown below in the following runs while a leak test was made. This also insured sufficient time for the plating chamber to thoroughly degas.

, Concurrently with the above preparation of the plating chamber, the vapor generation means was prepared for the plating operation. In the following runs the plating compound consisted of solid trimethylamine aluminum hydride complex which was placed Within the vaporization vessel which was in turn connected to the conduit leading to the plating chamber. The vessel, flow meter, and related conduit, all of which were isolated from the 7 plating chamber during the preliminary start-up procedure by having the control valve means in an off position, were then purged of residual air similarly to the manner of purging the plating chamber.

To start the plating operation, with the chamber open 7 to the vacuum pump and condenser, the plating comto that of Example I, but vertically oriented. Insteadof' pound feed valve was opened. In the following runs flow to the plating chamber was controlled by manipulation of the control valve'which in turn dictated the deposition rate for a given set of temperature-pressure conditions.

Upon, achieving the desired thickness of coating, heating of the plating compound was discontinued and the feed control valve was closed. Heat was. continued to be applied. to the substrate for a short period of time to insure they complete decomposition of any remaining vapors. This prevented deposition of aluminum under con EVALUATION OF COATINGS BY DIFFUSION AND OXIDATION TESTS Aluminum Coated Difiusion Oxidation High Temp. Diffusion Wt. Gain Wt. Gain Spelling Alloy Substrate. Conditions Mg. MgJCml Mg. MgJCm.

Cobalt base l,400 F./16 hr- 1.9 0.28 5.6 0. 82. N0118- Do 1,400 F116 hr... 5.8 0.87. 10.1 1.6i Very small.

1,950 F./1 hi1--. Y Nickel base 1,400 F./16 hr 2.9 0.45 4. 0.55 None.

1,950 F./l6 hr.-- l,600 F./6 hr 5.7 0.82 17.7 2.5 Do. Do 1,60 F./2 hr 7.0 1. 06 12.2 1.9 Very small.

1,800" F./2 hr 2. 2 0.32 6.0 0.88 None. D0 1,950 F./2 hr 15.8 27.2 4.1 smell.

ditions other than those specifically maintained during the plating operation. After cooling, the coated high temperature alloy substrates were removed from the plating chamber and subjected to the evaluation which follows to determine their suitability for high temperature applications.

In the following Runs 1 and 2, the plating gas was prepared by heating the trimethylamine complex of aluminum hydride dissolved in toluene. In Runs 3 and 4, the same compound was employed except it was heated in its solid state to generate the desired vapors.

Run 1 Run 2 Super alloy Cobalt base Cobalt base. Substrate temperature 176 0. avg 161 C. avg. 1 Compound temperature 103 C. avg 105 0. avg. Plating pressure 0.5 mm. Hg avg 0 4 mm. Hg avg Plating time. 120 minutes- Coating thicknes 2.1 mils 2.5 mils. Coating appearance Gray-white, satin Gray-white, satin finish. finish.

Run 3 Run 4 Super alloy Cobalt base Nickel base. Substrate temperature 180'C.'avg 177 0. avg. Compound temperature- C. avg 48 0. avg. Plating pressure 1 0.6 mm. Hg avg 0.5 mm. Hg avg. Plating time... minutes- 90 minutes. Coating thieknes 2.1 mils 2.2 mils. Coating appearance ray-white, satin Gray-white, satm finish. finish.

In the following run, the plating agent is the bis-trimethylamine complex of aluminum hydride in solution.

In the following run, the triisopropylamine complex of aluminumhydride is employed as the plating agent.

Run 6 Super alloy Tantalum base. Substrate temperature 160 C. avg. Compound temperature C. avg. Plating pressure 0.1 mm. Hg avg. Plating time Iminutes. Coating thickness 2.3 mils. Coating appearance Gray-white-satin finish.

In the following example, a mixture of the trimethylamine and bis-trimethylarnine complexes of aluminum hydride is employed as the platingagent.

Compound temperature In the above diffusion evaluations, even where, spalling was evaluated as small or very small, such spelling-wasstill less than with diffused aluminum coatings appliedby other known commerical vapor plating processes on the same substrate.

Uncoated control specimens were also tested for oxidizability to serve as a, basis upon which theperformance of identical specimenshaving the unique aluminum coat.-

ing of. this invention could becompared. A typical unr coated cobalt base alloywhen oxidized had a weight gain.v

of 79.3-mg. or 10.2, mg./cm. which is approximately three times as great as the poorest performing coated specimen.

Using the apparatus of Example I, the following additional runs were made:

aluminum hydride).

Substrate 25 C. Substrate temperature Lintless paper. Result C.

Adherent,- metallic coating.

Run 4; po d H3 )3N]2: H3, (bi

' (trimethylarnine) come plex of aluminumv hydride). Compound temperature 25C. Substrate Rayon taflfeta. Substrate temperature 160 C. Result Adherent, metallic coating.

1 1 Run Compound (CH N-AlH (trimethylamine complex of aluminum hydride).

The apparatus of Example I is employed with the exception that in place of the resistance heating means, a battery of infrared lamps is placed circumferentially around the outside of the plating chamber.

Run 1 Compound (C H N'AIH (triethylamine complex of aluminum hydride).

Compound temperature 75 C.

Substrate Mild steel.

Substrate temperature 200 C.

Result Adherent, metallic coating.

EXAMPLE V The apparatus of Example IV is employed. The substrate to be plated is brought to a temperature just below the decomposition temperature of the plating agent with the infrared heating and thereafter decomposition is effected with infrared rays. The following runs are made:

Run 1 Compound [(C H N] bis-triethylamine) complex of aluminum hydride).

Compound temperature 50 C.

Substrate Rayon.

Substrate temperature 100 C.

Result Adherent, metallic coating.

Run 2 Compound [(CI-I N] -AIH (bis-(triylamine complex of aluminum hydride).

Compound temperature 40 C.

Substrate Cotton flannel.

Substrate temperature 160 C.

Result Adherent, metallic coating.

The apparatus employed in the following examples was that of Example II.

The high temperature super alloy members to be coated were first degreased by washing with 1,1,2-trichloroethylene Whereafter they were vapor blasted. Following this they were rinsed well in cold water and then rinsed in a percent sodium hydroxide solution followed by a hot Water rinse. They were then quickly pickled by exposure to a 10 percent HCl solution followed by a final water rinse. After drying by exposure to hot air they were mounted in the plating chamber and placed under vacuum.

The plating compound employed in Examples VI, VII, VIII and IX was the trimethylamine complex of aluminum hydride dissolved in toluene except in Examples VIII and IX where the compound in its solid state was employed. The plating agent contained in the vaporization chamber was continually heated throughout the plating operation. Flow control means was provided between the vaporization chamber and the plating chamber whereby the rate of deposition was controlled.

12 EXAMPLE VI Super alloy Cobalt base. Substrate temp. 176 C. Avg. Compound temp. 103 C. Avg. Plating pressure 0.5 mm. Hg Avg. Plating time minutes. Coating thickness 2.1 mils. Coating appearance Bright gray-white with a satin finish. Diffusion temp. and time 1400 F./l6 hrs. Ditfusion atmosphere Air. Diffused coating thickness 3.4 mils. Dilfusion wgt. gain 0.87 mg./cm. Final appearance Medium gray. Coating thickness after oxidation 5.5 mils.

at 2000 F EXAMPLE VII Super alloy Cobalt base. Substrate temperature 181 C. Avg. Compound temperature 105 C. Avg. Plating pressure 0.4 mm. Hg Avg. Plating time minutes. Coating thickness 2.5 mils.

Bright gray-white with Coating appearance a satin finish. Diffusion temp. and time 1400 F./ 16 hrs. Diifusion atmosphere Air. Diffused coating thickness 3.7 mils. Diffusion wgt. gain 0.46 mg./cm. Final appearance Medium gray. Coating thickness after oxidation at 2000" F. 6.5 mils. Oxidation wgt. gain 1.29 mg./cm.

EXAMPLE VIII Super alloy Cobalt base. Substrate temperature C. Avg. Compound temp 50 C. Avg. Plating pressure 0.6 mm. Hg Avg. Plating time 90 minutes. Coating thickness 2.1 mils.

' Bright gray-white with Coating appearance a satin finish. Diffusion temp. and time 1400 F./ 16 hrs. Diffusion atmosphere Air. Diffused coating thickness 3.5 mils. Diffusion wgt. gain 0.28 mg./cm. Final appearance Medium gray. Coating thickness after oxidation at 2000" F. 6.5 mils. Oxidation wgt. gain 0.82 mg./cm.

EXAMPLE IX Super alloy Nickel base. Substrate temperature 177 C. Avg. Compound temp 48 C. Avg. Plating pressure 0.5 mm. Hg Avg. Plating time 90 minutes. Coating thickness 2.2 mils.

Bright gray-white with Coating appearance a satin finish.

1400 F./16 h-rs.- Dilfusion temp. and time 2000 F./1 hr. Diffusion atmosphere Air. Diffusion time 1 hour. Diffused coating thickness 3.6 mils. Diffusion wgt. gain 0.34 rng./cm. Final appearance Medium gray. Coating thickness after oxidation at 2000 F. 8.0 mils. Oxidation wgt. gain 0.85 mg./cm.

In the following examples the plating agent .is bis-tri- EXAMPLE X Super alloy Nickel base.

Substrate temperature 169 C. Avg.

Compound temp 107 C. Avg."

Plating pressure 0.2 mm. Hg Avg.

Plating time v110 minutes.

Coating thickness 1.9 mils.

- Bright gray-white with Coating appearance a satin finish.

Diffusion temp.and time 2250 F./ /2 hr.

Diffusion atmosphere Argon.

Diffused coating thickness 3.3 mils.

Diffusion Wgt. gain 0.47 mg./cm.

Final appearance Medium gray.

Coating thickness after oxidation 15.

at 2000 F. 6.1 mils.

Oxidationwgt. gain 1.08 mg./cm.

' In the next example, the triisopropylamine complex of aluminum hydride is employed asthe plating agent.

EXAMPLE XI Super alloy Tantalum.

Substrate temp 180 C. Avg.

Compound temp 110 C. Avg.

Plating pressure 0.1 mm. Hg Avg.

Plating time 130 minutes.

Coating thickness 2.3 mils.

Bright gray-white with Coating appearance a satin finish.

Dilfusion temp. and time l700 F./5 hrs.

Diffusion atmosphere Hydrogen.

Diffused coating thickness 3.6 mils.

Final appearance Medium gray.

Coating thickness after oxidation at 2000 F. 7.1 mils.

Inthe following example a mixture of trimethylamine and bis-trimethylamine complexes of aluminum hydride is employed as the plating agent. 1

EXAMPLE XII Super alloy Columbium.

Substrate temp. 160 C. Avg.

Compound temp. 100 C. Avg.

Plating pressure 0.3 mm. Hg Avg.

Plating time 130 minutes.

Coating thickness 2.3. mils.

' Light gray with. a

Coating appearance satin finish.

Diffusion temp. and time. 1400 F./. 16 hrs.

Diffusion atmosphere Argon..

Diffused coating thickness 2.9 mils.-

Fiualappearance Dark gray.

Coating thickness, after oxidation. at

2000 F. 4.9. mils. 55.

EXAMPLE XIII Highly. uniform coatings using trimethylamine complex of aluminum hydride were achieved inv the following manner. Parts-to-be-coated were placed-inside a. cir.- 6Q

cular tunnel and slowly revolved in the tunnel on a rotary table forming the floor of the tunnel. A vapor plating stream passed at an entry port into the tunnel was split, part flowing concurrently with the revolving parts (e.g.,

clockwise in the tunnel) and the balance flowing counter- 5:

1.4 A plating time of from 30 minutes to 2 hours was utilized to deposit 0.5 mil to 1, mil of aluminum. An average of; about 30 to 40 percent of the-aluminum feed was deposited upon the parts. Due to the short length of: the,

tunnel, 30 to 40 percent of unreacted'plating compound;

passed out the exit of the tunnel. About, 30 percent .of the aluminum Was, placed on the tunnel walls and holding means. Using this apparatus and process, a. /2 inch diam.-

eter threaded Parker type steel, tube fitting; was given. av coating thickness of 0.55 mil witha spread of;o,nly- 20.05;.

mil, including the root of the threads. I I

The tertiaryamine complexes of. aluminum. hydride. which are. employed as platingagents in this inventionare those having the following illustrative formulas'( s)x wherein x is an-integer having a value of 1 or- 2- and; R is selected from the group consisting of alkyl, alkenyl,

whereby a high degree of purity control is easier to 0312-- tain. The trimethylamine and bis-trimethylamine. complexes of aluminum hydride, as well as mixtures thereof-,. areespecially preferred since they have the. best all-i around stability.

Exemplary of the tertiary amine complexes: of, alumi-. num hydride that can be employed in the instant invention arethose wherein the tertiary amine constituentis for example: trimethylarnine; triethylamine; tri-n-propylamine; triisopropylamine; tri-n-butylamine; tri-sec-butylamine; tri-tert-butylamine; tri-isobutylamine; tripentyla: mine trihexylamine; methyl diethylamine; n,-propyldime-. ethylamine; isopropyl diethylamine; n-butyldiethylamine; isobutyldimethylamine; triethenylamineg triisopropenylar Ininetriethynylamine; propynyldimethylamine; tricyclopentenylamine; tribenzylamine; tristyryl'amine; benzyldistyrylamine; dimethylbenzylamine; methylbutenylbenzylamine; tri-phenylamine tritolylamine; tricumenylamine; and; the like. The above tertiary amines can contain substituted hydocarbon groups, such as trichloromethylamine; tri(1- iodo-2-methylpropyl)amine; trichlorobenzylamine; and" the like. Such substituted tertiary amines offer no particular technical advantage over theabove, preferred: compounds and are most costly because they are moredifiicult to prepare and require the use of more expensive raw ma,- terials for their synthesis.

Thev above compounds are decomposed in an inert atmosphere to avoid oxidation of the coating, substrate, or.- plating agent during theplating operation. The presence:

of oxides in the coating is detrimental and objectional for" many applications, There are several techniques wherebyv an inert atmosphere can be provided. One is to continually purge the plating system with an inert gaswhichis compatible with the alloy base material, plating compound, and. effected coating. Examples of suitable inert gases are nitrogen, hydrogen, helium, neon, argon, krypton, xenon, gaseous aliphatic hydrocarbons, and the like.

A more preferred method to provide an inert atmosphere is by conducting this process under reduced pressure. By evacuating the plating system undesirable con taminants willbe removed. Additionally, theme of reduced pressure during the plating operation will assure expeditious removal of the lay-products of decomposition. .1

Low pressure operation also makes it. easier to maintain-a. fairly constant vapor flow. rate. This method ofproviding an. inert environment is preferred sinceit'oifers an economic advantage in that the expense of. an inert medium can be avoided, althoughsuchavoidance-is not essential.

Indeed when a carrier gas is used, it is preferred to .use .it at" L subatmospheric pressures. The carrier gas is preferably provided by dissolving the plating compound in a hydrocarbon solvent whereafter the resultant solution is heated to generate the desired plating vapors simultaneously with the carrier gas. Aromatic hydrocarbons, especially benzene and toluene, are preferred solvents for the preferred compounds of this invention, although other solvents such as the alkanes, cycloalkanes and the like can be employed as long as the plating agent is soluble therein. This manner of supplying a carrier gas offsets the pyrophorosity of the plating compounds and also reduces their concen tration in the resulting vapors so that a uniform rate of vaporization of the plating compound is more readily assured. However, as mentioned supra a distinct advantage of the instant process is that a carrier gas is neither mandatory nor preferred.

When employing reduced pressure, it is preferred to operate at pressures less than millimeters of mercury absolute, especially between 0.1 and about 5 millimeters of mercury. Operating within these pressures assures rapid removal of volatile by-p-roducts and trace impurities that may inadvertently leak into the system and also assures desirable flow rates of the plating compound without resort to the use of high vaporization temperatures to generate the plating gas.

The substrate, i.e. the object or material upon which the aluminum derived by decomposition of the aforesaid amine complexes of aluminum hydride is to be plated, or coated, can be any substrate which is stable at the conditions employed to effect decomposition of the amine complex of aluminum hydride. By stable is meant that the substrate does not decompose under the decomposition conditions utilized to deposit the aluminum plate or coating. Thus, for example, if thermal decomposition of an amine complex of aluminum hydride is the decomposition technique being employed, and the temperature necessary to effect such decomposition is high enough to cause the substrate to begin to soften, the substrate is considered stable if it still retains its shape enough to effectively support the aluminum deposit being laid down.

The substrates employed in this invention range all the way from paper and fibers to metals and metalloids, especially of Groups III through VIII of the Periodic Table of the Elements (Fisher Scientific Company, 1955), to ceramics and refractory materials. When the term fiber is employed herein the fiber can be in the form of filaments, fabrics, strands, cloths and the like.

The metals which are employed as substrates in the process of this invention are those metals which are solid at the conditions employed to plate the aluminum metal upon the metal substrate. Thus it can be seen that such metals as mercury, which is a liquid under normal conditions, is not contemplated as a substrate within the scope of this invention. Additionally, metals which would react with the plating gas to form undesirable coatings and prevent deposition of the aluminum metal are likewise not attractive. Thus the metal substrates of this invention include for example: scandium, yttrium, lanthanum, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, technetium, rhenium, boron, indium, tellurium, germanium, silicon, tin, lead, antimony, bismuth, iron, cobalt, nickel, ruthenium, rhodium, osmium, iridium, beryllium, magnesium, copper, zinc, and the like.

One of the noteworthy advantages of the subject invention is that for the first time there is made available a very practical method for depositing aluminum via vapor deposition employing heat decomposable compounds, at temperatures below 220 C. Thus, such materials or substrates as paper, cotton, wood, and the like can be aluminum coated by the process of this invention. Furthermore synthetic fibers and fabrics, such as rayon, dacron, fiber glass, and the like, can also be employed as substrates. Aluminum can also be plated on plastic substrates by the process of this invention, such as polyethylene, polyvinyl chloride, Tefiona polytetrafiuoroethylene, Kel-F a polych-lorotrifiuoroethylene, Delrin-a polyformaldehyde, polystyrene, and the like. Ceramics, cermets, and nonmetallic refractories also form suitable substrates which can be employed in the process of this invention. These materials include carbonaceous materials such as carbon and graphite powders and shapes, as well as refractory materials like carborundum, carbides, such as silicon carbide, vanadium carbide, tantalum carbide, uranium carbide and the like. This would also include nitrides like boron nitride, titanium nitride, niobium nitride, tantalum nitride and other similar nitrides and oxides such as aluminum oxide, chromium oxide,-silicon oxide, zirconium oxide and the like.

For certain applications the substrate to be plated is preferably subjected to an initial preparation step. This is especially desirable in the case of metal substrates which are seldom commercially supplied free from contaminants. In other words, the degree of adherence achieved through this unique vapor plating process can in certain instances be further improved by appropriate metal surface pretreatment. A suitable metal surface preparation is achieved through degreasing with -a solvent such as 1,l,2-trichloroethylene or the like followed by light sand blasting. It is well known that vapor blasted coatings exhibit better adherence when they are effected on slightly uneven surfaces, such as those created by sand blasting as opposed to a highly polished surface. However, other desirable methods can be employed, such as acid pickling. On some substrates such as graphite and ceramics where the surface is already non-uniform, it is feasible only to degrease the surface preparatory to plating. Other well known methods of substrate pretreatment can be employed in lieu of the above.

Heating of the substrate can be accomplished by many well known means. Generally, resistance heating, infrared heating, or induction heating means are provided since they are particularly suitable for heating materials without contaminating them. The latter type of heating is especially attractive where the temperature of the substrate is to be varied during the plating operation. Quite often, the nature of the substrate will influence the selection of a particular type of heating means. For instance, induction heating is preferred when working with small intricate substrates which do not possess a high thermal conductivity. Flat substrates, such as metal plates, are generally heated by conduction from resistance heating apparatus such as a hotplate. The determination of a suitable heating means is additionally influenced by the plating environment in relation to such factors as to whether the plating'operation is a continuous or batch process, the number of substrates to be plated in unit time, and additionally to the physical and structural arrangement of the plating equipment. These factors are a matter of design within the abilities of one skilled in the art.

The fiow rates are not critical. 'However, extremely high or low rates make it difiicult to maintain coating uniformity without taking extra precautionary measures. A preferred plating rate is from 0.5 to about 20 mils per hour, especially between about 1 and about 5 mils per hour. These plating rates minimize equipment and operation costs.

The aluminum coated materals produced by the process of this invention find utility in a wide range of applications. Thus, when aluminum is deposited on the fibers of fabrics the heat reflectivity thereof is substantially increased. -Fabrics exhibiting such heat reflectivity are primarilyuseful in clothing having increased warmth characteristics, but can be employed in many other uses as, for example, in draperies, fire fighting equipment, and the like.

Another use of aluminum coated fibers is in the preparation of tire cords. Aluminum has good rubber adhesion characteristics, and can be plated on fibers, such 1 7 as fiberglass, which exhibit excellent strength characteristics, and other desirable properties but which do not have the necessary rubber adhesion, to render such materials extremely useful as a tire cord. Aluminum coated cotton also exhibits excellent tire cord properties.

The most significant contribution made possible by this invention is its application for the protection of metallic members exposed to corrosive medias. For example, there is a tremendous demandin the aircraft industry today for coated fasteners, especially aluminum coated, which would give better service for many applications than cadmium plated fasteners. This unique invention can fulfill that need plus many more. Another acute need in the aircraft industry for aluminum coatings is for the protection of refractory mate-rials exposed to high temperature oxidizing conditions. For many applications in this area, it is especially important that a high purity aluminum coating be achieved without subjecting the substrate .to temperatures that would alter any particular physical properties previously induced in it.

The high temperature super alloys embraced by this invention comprises those having as a principal or base constituent a metal selec-ted from the group consisting of cobalt, nickel, iron, niobium, tantalum, and titanium, whose oxides generally are not stable at elevated temperatures in the neighborhood of or greater than 1800 F., aside from other properties which may bar their use. A significant feature of the present invention is that the entire operation can be conducted at temperatures much lower than those presently employed in the art, whether it be by present day pack techniques or vapor plating. This significant aspect makes it possible to coat many high temperature super alloy gas turbine components which have received previous special heat treatment, such as tempering followed by machining to close tolerances. A distinct advantage of this invention is its superior throwing power whereby irregular or complex shaped gas turbine components having inaccessible areas can be readily plated.

Typical well known high temperature super alloy compositions are presented below.

COBALT BASE ALLOYS Alloy 1 1 Alloy 2 2 Alloy 3 Alloy 4 Carbon 0. 78-0. 93 3 20 Manganese. Chromium. Silicon.

Molybdenum" Niobium 1 U.S. Patents 2,974,036 and 2,974,037.

2 U.S. Patent 3,026,199. 3 Max. 4 Balance.

NICKEL BASE ALLOYS Alloy 5 1 Alloy 6 2 Chromium Cobalt Zirconium Man 'ma Silicon Columbium- Nickel 1 Trade Name: Waspalloy.

2 U.S. Patent 2,688,536.

3 Trade Name: Inconal Alloy 713C. 4 Max.

5 Balance.

Other suitable alloys and their nominal alloy percent- 18 age content that can be coated pursuant to this invention are:

Udimet 500: 0.08 C, 0.75 Mn, 0.75 Si, 19 Cr, 19.5 Co,

4.0 M0, 2.9 Ti, 2.9 A1, 4 Fe, 0.01 other, Ni bal.

Udimet 700: 0.15 C, 15 Cr, 18.5 Co, 5.2 M0, 3.5 Ti,

4.25 Al, 1.0 Fe, 0.1 other, Ni bal.

IN-lOO PDRL: 0.18 C, 10 Cr, 15 Co, 3 Mo, 5 Ti, 5.5

A1, 1.0 Fe, 0.015 other, Ni bal.

W152(C): 0.45 C, 0.50 Mn, 0.50 Si, 21 Cr, 1.0 Ni, 11W,

2.0 Cb, 2.0 Fe, Co bal.

Cb: 0.036 O, 0.019 N, 0.024 C, Cb bal.

FS-82: 33 Ta, 0.75 Zr, Cb bal.

100 Mo: 0.01-0.03 C, Mo bal.

Mo-0.5 Ti: 0.5 Ti, 0.02-0.05 C, Mo bal.

Ta-lO W: 10 W, 0.0045 0, 0.0015 N, 0.001 C, Ta bal.

100 W: 0002-0005 0, 0.0015-0.004 N, 00004-0005 C,

W bal.

For additional base materials suitable for use in the instant invention, reference can be made to the article What Alloy Shall I Use for High Temperature Applications, Metal Progress, October 1961.

The alloy base material to be coated is preferably subjected to an initial surface preparation step. A suitable cleaning procedure is to first degrease the material with a halogenated solvent such as 1,1,2-trichloroethylene or the like followed by vapor blasting with a fine grit. Cleaning by pickling with dilute acid can also be employed, after which the material is neutralized and then rinsed.

The novel and economical improved high temperature super alloys made possible by the instant invention fulfill a need for better high temperature materials for use in propulsion systems. This need is especially acute in both conventional development gas turbine engines where the lack of high strength components, such as vanes and blades with sufficient oxidation resistance, has limited operating temperatures and where higher temperatures are desired to achieve greater power and efliciency.

We have described what we believe to be the best embodiment of our invention. However, we do not wish to be confined within those embodiments in the enumerated examples which are only illustrative of our invention, but what we desire to cover by Letters Patent as set forth in the appended claims is:

1. In a process of depositing an aluminum coating on a substrate by contacting the substrate under a reduced pressure of below about 30 millimeters of mercury with vapors of an aluminum compound that decomposes to deposit the aluminum, the improvement according to which the aluminum compound is an amine complex of aluminum hydride.

2. The combination of claim 1 in which the substrate is heated to a temperature within a range of from about C. to about 250 C. and maintained within said range during at least part of the period while contacting said substrate with said vapors.

3. The combination of claim 2 in which the vapors are generated in a non-oxidizing environment at a temperature of from about 20 C. to about 80 C. and brought into contact with said substrate in a non-oxidizing environment.

4. The combination of claim 3 in which a desired thickness of coating is achieved at a rate of from about 0.5 to about 20 mils per hour.

5. The combination of claim 1 in which the substrate is heated to a temperature within a range of from about C. to about 200 C. and maintained within said range during at least part of the period while contacting said substrate with said vapors.

6. The combination of claim 5 in which the vapors are generated in a non-oxidizing environment at a temperature of from about 20 C. to about 80 C. and brought into contact with said substrate in a non-oxidizing environment.

7. The combination of claim 6 in which a desired thickness of coating is achieved at a rate of from about 0.5 to about 20 mils per hour.

8. In the process of coating an oxidizable high-temperature alloy with aluminum under a reduced pressure of below about 30 millimeters of mercury and then diffusing the aluminum from the coating into the surface of the metal to form a skin highly resistant to destructive oxidation, the improvement according to which the aluminum is deposited by vapor-plating from an amine complex of aluminum hydride and the diffusion is effected Without intervening treatment of the deposit.

9. The combination of claim 8 in which the substrate is heated to a temperature Within a range of from about 120 C. to about 250 C. and maintained within said range during at least part of the period While contacting said substrate with said vapors.

10. The combination of claim 9 in which the vapors are generated in a non-oxidizing environment at a temperature of from about 20 C. to about 80 C. and brought into contact with said substrate in a non-oxidizing environment.

11. The combination of claim 10 in which a desired thickness of coating is achieved at a rate of from about 0.5 to about 20 mils per hour.

12. The combination of claim 11 in which said alloy is heated to a temperature within the range of from about 1200 F. to about 2250 F. for a period of time within the range of from about /2 to 16 hours to diffuse the aluminum from said coating into said alloy.

13. The combination of claim 8 in which the substrate is heated to a temperature within a range of from about 160 C. to about 200 C. and maintained within said range during at least part of the period While contacting said substrate with said vapors.

14. The combination of claim 13 in which the vapors are generated in a non-oxidizing environment at a temperature of from about 20 C. to about C. and brought into contact with said substrate in a non-oxidizing environment.

15. The combination of claim 14 in which a desired thickness of coating is achieved at a rate of from about 0.5 to about 20 mils per hour.

16. The combination of claim 15 in which said alloy is heated to a temperature Within the range of from about 1200" F. to about 2250 F. for a period of time within the range of from about /2 to about 16 hours to diffuse the aluminum from said coating into said alloy.

17. The combination of claim 1 in which the pressure at which the depositing is effected in less than 10 millimeters of mercury.

References Cited UNITED STATES PATENTS 2,490,543 1/1919 Robertson 14-8-1l5 2,643,959 6/1953 Fischer 117-107.2 2,880,115 3/1959 Drummond 117-50 2,970,065 1/1961 Greene et al.

OTHER REFERENCES Zeitschrift Fur Organaische Und Allegmere Chem-a1, vol. 272, pages 221 and 226 relied on, 1953 (translation relied on), 117-107.

ALFRED L. LEAVITT, Primary Examiner.

A. GOLIAN, Assistant Examiner. 

